Non-destructive inspection and characterization of track bed with microwaves

Railway ballast is not indestructible and requires regular inspections and maintenance. Failure to maintain the ballast integrity could affect productivity and potentially lead to derailment, with possible catastrophic consequences. Traditional methods of inspecting ballast involve potholing random areas along the railway track. With this method, sample cores are extracted from randomly selected areas and inspected for contamination. The disadvantage of this method is that the random areas selected or those assumed to be suspect, may be integrally sound, whereas those areas in-between the sampled areas may be fouled or contaminated, making the reliability of the maintenance program questionable. The potholing method is also costly, time consuming and considered a destructive activity. Non-destructive inspection technologies are consequently very attractive; and particularly amongst these, the use of microwave systems is becoming popular all around the world. However, the inspection of the track bed imposes a particular set of constraints on the design of an effective sub-surface radar system (commonly called Ground Penetrating Radar or GPR). Basically, collection speed must be high enough not to interrupt the regular traffic on the track; then, suitable sensors (transmitter and receiver) have to be implemented and used to inspect the ballast bed down to the required investigation range (usually 2 meters below rail level). Moreover, the design of such system cannot disregard the laws of the physics governing the propagation of a radiofrequency signal in multi-component material (ballast); for instance, the choice of the radar working frequencies has to be taken by considering the scattering characteristics of the target as well as the phenomenology that needs to be highlighted.